1. Field of the Invention
The present invention relates in general to flow meters which are designed to measure the flow of liquid or gas through a conduit. More particularly, the present invention is directed to Coriolis effect mass flow meters and the displacement sensors which measure the oscillations induced by the flow tubes which form an integral part of such flow meters.
2. Description of Related Art
A number of different instruments are available for measuring the flow of mass through a conduit. One type of meter measures mass flow as a function of Coriolis forces generated by mass flow through an oscillating tube. These types of flow meters are generally referred to as Coriolis mass flow meters. Coriolis mass flow meters are described in U.S. Pat. Nos. 4,127,028; 4,187,721; and 4,843,890. The contents of these patents are hereby incorporated by reference.
Coriolis mass flow meters are useful for measuring the mass flow of gases, liquids, gels and solid suspensions. During operation, the angular momentum of the flowing mass is changed by causing it to flow around a section of pipe or flow tube which is typically in the shape of a "U". The flow tube is vibrated or oscillated at a frequency which subjects the fluid to a Coriolis acceleration. The resulting forces angularly deflect or twist the flow tube with a magnitude that is directly related to the quantity of mass flowing through the tube. The amount of deflection is sensed by a displacement sensor and converted to a quantity which is directly proportional to mass flow rate.
Many Coriolis mass flow meters utilize a double flow tube design. The double flow tube designs have the attendant advantage of providing a greater accuracy and precision in measuring the mass flowing in the system. These flow meter designs consist of two flow tubes, each of which carries one-half of the mass entering the flow meter. Two displacement sensors are placed so that each sensor is coupled between the flow tubes. The two flow tubes are vibrated or oscillated at a frequency near their natural frequency. The oscillating flow tubes subject the flowing mass to a Coriolis acceleration which, in turn, generates Coriolis forces which angularly deflect the tubes. The displacement sensors measure the deflections in the leading and trailing edges of the tubes. The mass flow rate is determined by measuring the angular deflections between the flow tubes and calculating the phase difference between the two.
The displacement sensors typically used for measuring the oscillations of the flow tubes are magnetic position detectors. These detectors measure small mechanical deflections of the flow tubes by sensing variations in electromagnetic fields. One problem associated with the use of magnetic position detectors is their sensitivity to electromagnetic interferences. The mass flow measurements obtained from these magnetic position detectors become unreliable when they are subjected to electromagnetic fields which are not related to the flow tube movement.
Another problem with Coriolis mass flow meters which utilize magnetic position detectors is the requirement for electrical wiring and the associated electrical power. Coriolis mass flow meters which utilize such magnetic position detectors in conjunction with measuring the mass flow of flammable or explosive materials present potential explosion hazards in the event an electric spark ignites the flammable or explosive liquid.
In many instances, it is desirable to measure the flow of liquids at high temperatures. The Coriolis mass flow meters used in such high temperature situations must be both mechanically and thermally rugged. Unfortunately, flow meters which utilize magnetic position detectors are not well suited for such high temperature applications.
In view of the above considerations, there presently is a need for Coriolis mass flow meters that have displacement sensors which are immune to electromagnetic interferences. Further, the sensors should avoid or minimize explosion hazards and be both mechanically and thermally rugged.